Infrared spectroscopy for determining a concentration of individual gas components has previously been described. The most common methods relate to the Fourier transform infrared spectrometer or the non-dispersive infrared spectrometer. With the development of compact high-power semiconductor lasers, gas analyzers based on the laser spectroscopy have been established to an increasing extent. New laser types, such as quantum cascade lasers, revolutionize laser spectroscopy in the medium infrared range.
All these analyzing methods rely on specific frequency ranges being absorbed during the irradiation of a sample gas with infrared beams. The infrared radiation lies in the range of the oscillation level of the molecular bonds which are induced to oscillate by the absorption. A prerequisite therefor is a dipole moment which is either already present or which is generated in the molecule. The different oscillation states cause absorption losses of the infrared radiation of different optical frequencies. The spectrum in the transmission thus contains individual absorption lines characteristic of the gas so that the sample gas can be examined for the presence of concrete molecules, and their concentration in the sample gas can be determined.
A quantum cascade laser can in particular determine the presence and concentration of pollutant molecules in the exhaust gas of internal combustion engines, such as dinitrogen monoxide, nitrogen monoxide, nitrogen dioxide, carbon dioxide, carbon monoxide and ammonia.
Common laser-spectroscopic systems comprise a laser as a radiation source, the radiation of which is conducted into a gas cell via an optical path. The beam is repeatedly reflected in the gas cell via a suitable mirror configuration. A sample gas flow is introduced at the same time into the gas cell, wherein the radiation of the laser penetrates the sample gas flow and excites the molecules corresponding to the optical frequency. The respective frequency is absorbed due to this excitation energy. The intensity of the transmitted beam decreases at this point in the spectrum. The absorption itself is not defined exactly, but is subject to a broadening due to temperature and pressure changes. The beam having its spectrum changed in this manner exits the measuring cell and impinges upon a detector via which the changed frequency band is evaluated, thus allowing the presence of specific substances and their concentration to be determined. The sample gas flow is usually delivered via a downstream vacuum pump.
The absorption characteristic in the spectrum is evaluated and/or analyzed when determining concentration. This characteristic is generally referred to as the line spectrum of the absorbing gases. It has turned out, however, that the line shape in this spectrum depends on pressure and temperature. For the purpose of evaluation, these parameters must therefore either be kept constant or must be continuously metrologically detected and offset. The gas is therefore conditioned and the pressure and temperature are kept constant in order to increase measuring accuracy. Both the measuring cell and the supply line of the measuring gas must be heated for tempering purposes. The occurrence of temperature gradients must therefore be prevented during the entire sampling process to avoid gas entrainment effects and thermal turbulences which would affect the absorption behavior of the laser radiation during its passage through the medium.
For carrying out such a conditioning of the sample gas, it is common practice to heat the gas cell and the sample gas in advance to a specific temperature. Heating hoses are, for example, used to preheat the sample gas, as is described in EP 2 388 570 A1. The measuring cell as well as the sample gas in the supply hose are heated, for example, to 191° C. While this design helps to improve measuring results, a problem associated with using two different temperature probes and temperature controllers arises so that measuring errors due to temperature gradients occurring between the gas cell and the sample gas flow cannot be completely precluded.